It is known that water-soluble epoxy resin derivatives can be used in aqueous thermosetting coating systems, for example, for electrodeposition onto metals. British Pat. No. 1,556,201, for example, discloses resinous compounds prepared by reacting in the temperature range of from 100.degree. to 150.degree. C. hydroxyl-containing resinous intermediates with sufficient polycarboxylic acid anhydride to produce a resinous compound having an acid value of at least 35 mg KOH/g. The resinous intermediate is prepared by reacting below 150.degree. C., a polyglycidyl ether of a dihydric phenol with a hydroxyalkane monocarboxylic acid or a mixture thereof with a saturated aliphatic dicarboxylic acid having 4 to 12 carbon atoms per molecule. The intermediate is essentially carboxyl-free by the choice of a proper equivalent ratio of components, the use of rather low temperatures, and the use of a catalyst promoting the esterification of epoxy groups with carboxyl groups. The final resinous compounds are then diluted with a lyotropic solvent such as a glycol ether and neutralized with a base. These compositions can then be further diluted with water to make an aqueous solution for an electrodeposition bath. As the solids content in such a bath is rather low (at most 20%w solids) this procedure is quite satisfactory. However, difficulties arise when the procedure has to be adapted for aqueous lacquers and paints where organic solvents are not desired, and a more concentrated aqueous binder system is required, as for application by spraying and dipping. When not diluted, the resinous compounds as disclosed have usually melting points above 100.degree. C., and the viscosities before and after neutralization are too high for easy handling without dilution with a solvent. Furthermore, at room temperature the neutralized aqueous systems form usually a stiff paste in concentrations above 30 weight percent of binder.
It is further known that aqueous dispersions of thermosetting binders can be used in coating compositions.
Aqueous dispersions, in which the binder is dispersed in water instead of dissolved, have the advantage that higher binder concentrations can be used without viscosity problems, and that it is easier to evaporate the water in the coating from a dispersion than from a solution. A drawback, however, is usually that surface-active agents or dispersion stabilizers have to be added, which additives have generally an unfavorable effect on the properties of the hardened coating in that they tend to increase the water sensitivity.
Aqueous dispersions of thermosetting compounds can be made in general from resinous compounds having acid values below about 30, by neutralization and dilution with water. However, the type of resinous compound described in British Pat. No. 1,556,201, even with acid values below 30, can hardly be used for industrial production of aqueous dispersion paints. An improvement has now been found which allows this type of resinous compound to be used for the preparation of aqueous dispersions, and furthermore, provides processing advantages for the preparation of aqueous systems for use in electrodeposition. The improvement, from the point of view of starting materials, is the replacement of the saturated aliphatic dicarboxylic acid having up to 12 carbon atoms per molecule by dimerized unsaturated fatty acids, and from the point of view of the product, the use of a polycarboxylic acid anhydride in an amount to produce a resinous component having an acid value from about 5 to about 35.